If your credit card is being declined upon checkout, the first thing to do would be to try and make your purchase again while carefully double checking your card details. Your CVC and address are the two most common issues that get flagged as incorrect.
If you're getting an error code upon checkout, such as 'Error 000027', this may mean that your credit card type is not supported by the organizer of the event that you're trying to buy for. Some venues do not accept Visa Debit or American Express for example. In this case you will need to purchase with another credit card to get your order to go through.
Finally, if you are completing the purchase and receive a "Successful" message on the top right of your screen, but no tickets end up in the "My Orders" section of your profile, your transaction may have been blocked by your bank. This can happen if you are making a large purchase, or one that seems out of place when compared to your buying habits. You'll need to call your bank and verify that it is you making the transaction in order to allow the purchase, after which you can buy again. We cannot create a charge without bank approval, so the first one will not go through even if it is showing on your statements as "Pending". You'll need to go through the checkout process again in order to get your tickets.